German Patent Disclosure Document 36 13 230 OMORI/MITSUBISHI (claiming the priority of Japanese application 83128/85 of Apr. 18, 1985) discloses a method of repetitive reproduction of video signals in which both the advancement speed of the magnetic tape and the rotation speed of the head drum or reading device are changed by a factor n with respect to the speed during recording. By reason of this simultaneous alteration of the magnetic tape advancement speed and the rotation speed of the reading device, the magnetic playback heads in a magnetic tape spooling system can read or sample the recorded tracks exactly and without signal losses and can read the recorded video signals without error.
In practice, however, the reduction of the rotation speed leads to a change in the frictional relationships between the tape and the reading device, so that the magnetic playback heads diverge from the recorded tracks and follow a curved path. This causes disturbances in the reproduced television images. Further, the change in relative speed causes a frequency transformation of the reproduced signal.
Conventional circuits, e.g. equalizers and clocking regenerators, cannot be used in this method and must be matched to the altered frequency spectrum of the reproduced signal. In this known method, video signals obtained by the magnetic reproduction heads are written into a memory and are repetitively read back out of the memory in time-compressed form.
Further, German Patent Disclosure DE-OS 35 17 317, SCHOLZ, teaches a method of reading digital signals recorded on oblique tracks, in which method the advancement speed of the magnetic tape is sensed and used to derive a drive factor which modifies the speed of a rotating magnetic head in such a way that, during playback, accurate tracking of the obliquely recorded signal is maintained. The data rate of the reproduced signal agrees with the data rate of the digital signal when it was recorded. However, even in this method, no constant frictional relationships exist, so the recorded track image cannot be retrieved without error.
Finally, German Patent Disclosure DE-OS 38 08 198, NOGUCHI/HITACHI (claiming the priority of Japanese application 56768/87 of Mar. 13, 1987) discloses an apparatus for recording and reproduction of a digital signal of magnetic tape. In the recording mode, the rotation rate of a rotating scanning device and the advancement speed of the magnetic tape is varied by a factor of n-times or 1/n-times the normal speed. In the playback mode, conversely, the rotation rate of the scanning device is kept constant and only the advancement speed of the magnetic tape is adjusted by the factor n or 1/n of the normal speed. For playback for the track image obtained during the recording mode, given n=2, it is necessary, first, to double the number of magnetic heads along the circumference of the rotating scanning device, and second, to dynamically guide the magnetic heads along the track position in the playback mode (ATF). Systems for dynamic guiding of the magnetic heads along tracks are, however, very expensive and are not adapted for guiding magnetic heads along a multiplicity of tracks.